interval::hull occludes non-member hull() functions

[byron-hawkins]

There are functions hull(a, b) and also a static member function interval::hull(T, T). I am using the interval class as a base class. Within my type that is derived from interval<T>, I find that it is complicated to invoke the non-member versions of the hull() function, because the compiler becomes obsessively glued to the static member function interval::hull(T, T). It effectively occludes the others. Conversely, the static member function serves no purpose, because it performs exactly the same operation as its non-static counterpart. It would be convenient to have this static member function removed.